A dielectric rod antenna has been well known in the art since the 1930's but there is little available information that enables one to decide which dimensions or dielectric material to choose in designing such an antenna. The dielectric material chosen has to have low loss and have suitable physical characterisitics as well as being relatively low in cost. However, the choice between a high dielectric constant material and one of a low dielectric constant material has heretofore been left unanswered.
It is well known that electromagnetic fields or modes can exist in a dielectric cylinder. Most useful among the modes and the one which is the subject matter of the present invention is the dominant or HE.sub.11 mode. This mode can exist alone bound to a uniform cylindrical dielectric rod provided the rod diameter (D), and the wavelength (.lambda.) satisfies the inequality D/.lambda..ltoreq.to 0.766/ (Er-1).sup.1/2 where (Er) is the relative dielectric constant of the rod with respect to its surroundings. If the rod remains uniform as in the case of a fiber optic cable, no radiation will occur. However, at the end of a finite rod, the fields of the HE.sub.11 mode can be treated as though they extend over an aperture. From this aperture, the farfield radiation pattern can be calculated as presented by Brown and Spector, "The Radiating Properties of End-Fire Aerials", proceedings IEE, Volume 104B, January 1957.
A common method in the prior art of exciting a dielectric rod is shown in FIG. 1 which is taken from the Antenna Engineering Handbook, H. Jasik, Editor McGraw-Hill New York 1961, and will be discussed more completely hereinafter. Typically, a rectangular or circular wave guide is enlarged into a rectangular or circular launching horn. The cylindrical dielectric rod, usually tapered at the inserted end for over a few wavelengths, is inserted into the horn as shown in FIG. 1. Typically, the insertion of the dielectric rod into the horn is as a secured wedge with the dielectric rod making interference contact with the adjacent portions of the waveguide horn. The problem with this construction is that the electromagnetic field configurations of the waveguide or horn are not the same as the field configurations needed to efficiently excite the dielectric rod. As a result, only part of the energy available from the waveguide is transferred to the dielectric rod. The rest of the energy is usually radiated in an uncontrolled manner, e.g. enlarged side lobes, over a wide angle of space. If the rod is being used as a transmission line, the radiation represents a loss of energy. If the rod is being used as an antenna, the loss of energy is even worse. The uncontrolled radiation adds in the farfield with the intended radiation of the antenna and produces a result that is for the most part undesirable, such as an increase in the beam width from the antenna. Similarly situations arise from other forms of excitation with modes between the transmission waveguide and the dielectric rod not matching, both in energy distribution and spatial distribution.
Accordingly, it is desirable to provide a method and apparatus for the efficient excitation of a dielectric rod wherein the energy transferred from the waveguide to the dielectric rod is maximized and wherein the farfield radiation pattern of the antenna is not encumbered with beam broadening or wasteful side lobes.